Antennas have evolved in a wide variety of types, sizes and degrees of complexity. The application, including operating environment, for which an antenna is intended determines the characteristics which the antenna must have. For example, communication between two fixed ground stations is most readily accomplished by aiming the stations' respective antennas toward each other in a non-dynamic relationship. Space and weight may not be limiting factors. Linear polarization, narrow beamwidth and narrow bandwidth may be satisfactory.
A fixed ground station can also communicate with a geostationary or orbiting satellite by aiming the antenna at the satellite and maintaining such relationship. In both applications, circular polarization, broader beamwidth and broader bandwidth may be desirable or necessary. It may also be desirable that the antenna have a directed or "scanned" beam with a relatively broad bandwidth. Further, for many such uses, it may be desirable for the ground station to assume a low profile and, in fact, be concealable.
A mobile ground application generally imposes significant size and weight restrictions on the antenna. Further, it may be particularly desirable that the antenna be concealable and yet be capable of physical rotation in order to remain "locked" onto a satellite while the vehicle is in motion.
Microstrip patch antennas have frequently been used when size, weight and low profile are important factors. The bandwidth and directivity capabilities of such antennas, however, can be limiting for certain applications. While the use of electromagnetically coupled microstrip patch pairs can increase bandwidth, full realization of such benefit presents significant design challenges, particularly where maintenance of a low profile and broad beamwidth is desirable.
The use of an array of microstrip patches can improve directivity by providing a predetermined scan angle. However, utilizing an array of microstrip patches presents a dilemma: the scan angle can be increased if the array elements are spaced closer together, but closer spacing can increase undesirable coupling between antenna elements thereby degrading performance.
Furthermore, while a microstrip patch antenna is advantageous in applications requiring a conformal configuration, mounting the antenna presents challenges with respect to the manner in which it is fed such that conformality and satisfactory radiation coverage and directivity are maintained and losses to surrounding surfaces are reduced.